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2604 M. Rapp and F. J. Lübken: Review of PMSE Fig. 4. Seasonal variation of the occurrence rate of PMSE with a SNR ratio larger than 10 dB after observations with the ALWIN radar on the North-Norwegian island Andøya during the years 1999, 2000, and 2001. This figure is reproduced from Bremer et al. (2003), copyright by the American Geophysical Union. scale between the inertial and the viscous subrange; see also Lübken et al., 1993b) and the radar Bragg scale. Using a typical kinematic viscosity for an altitude of 85 km of 1 m 2 /s, this estimate yields ɛ≈100 W/kg equivalent to a heating rate of dT /dt=90 000 K/d. This heating rate is three orders of magnitude larger than all other relevant heating and cooling rates at the same altitudes and is clearly unrealistic (e.g., Mlynczak, 2000; Lübken, 1997). Ever since the initial observations, it has hence been a major goal of both experimental and theoretical investigations on PMSE to resolve this paradox and identify the physical processes responsible for the creation of structures in the electron number density where under normal conditions they should not exist. In the following subsections, we present an overview of the milestones regarding the available observational work on PMSE forming the background of our current knowledge. Note that given the wealth of experimental investigations we are aware of the fact that the results presented can only be a selection of some of the relevant work in the field. For further reading we also refer to the excellent previous review articles by Cho and Kelley (1993) and Cho and Röttger (1997). 2.2 Climatology 2.2.1 Seasonal variation As already evident from the pioneer work by Ecklund and Balsley (1981), PMSE reveal a pronounced seasonal variation. Figure 4 shows the seasonal variation of the occurrence rate of PMSE as observed with the 53 MHz ALWIN radar (ALWIN = ALOMAR Wind radar; ALOMAR = Arctic Lidar Observatory for Middle Atmosphere Research) on the North-Norwegian island Andøya (69.3 ◦ N, 16.0 ◦ E) during the years 1999, 2000, and 2001 (Bremer et al., 2003; Latteck et al., 1999a). The main characteristics are the steep increase during end of May/beginning of June, a rather high level near 90% in the middle of June until the middle/end of Fig. 5. Contour plot of the degree of saturation S as a function of season and altitude (shaded areas) derived from a falling sphere temperature and density climatology and model water vapor mixing ratios from Garcia and Solomon (1994). In addition, relative occurrence probabilities are shown (normalized to 100% at the maximum) for PMSE (dashed line) and NLC (dotted line). This figure is reproduced from Lübken (1999), copyright by the American Geophysical Union. July, and a more gradual decrease during August. The observations shown in Fig. 4 are in general agreement with independent observations from different radars located at about the same geographical latitude (e.g., Balsley and Huaman, 1997; Kirkwood et al., 1998). Intrigued by the fact that the seasonal variation of the NLC occurrence rate is very similar (Gadsden, 1998), several authors have tried to relate the occurrence of PMSE to the occurrence of ice particles that may form in the period from end of May until the end of August because of the very low temperatures of the mesopause region (e.g., Lübken, 1999). Figure 5 shows a comparison of the NLC- and PMSE-occurrence rate as a function of season and the time and altitude range during which the degree of water ice saturation S is larger than 1 (Lübken, 1999). S is the ratio between the actual water vapor partial pressure and the saturation vapor pressure over ice which Lübken (1999) derived from a climatology of temperatures and air densities obtained with the falling sphere technique and water vapor mixing ratios from model simulations (Garcia and Solomon, 1994). The saturation water vapor pressure over ice was derived from the empirical formula of Marti and Mauersberger (1993). It is evident from this comparison that PMSE occur in exactly the height and time interval where ice particles can exist in the mesopause region and confirms earlier considerations that ice particles may play a decisive role in the creation of the radar echoes (Cho and Kelley, 1993; Cho and Röttger, 1997). Note that since the work of Lübken (1999) better data on both water vapor in the mesopause region and an updated empirical formula for the saturation vapor pressure have become available (Seele and Hartogh, 1999; Körner and Sonnemann, 2001; McHugh et al., 2003; Mauersberger and Krankowsky, 2003). However, including Atmos. Chem. Phys., 4, 2601–2633, 2004 www.atmos-chem-phys.org/acp/4/2601/
M. Rapp and F. J. Lübken: Review of PMSE 2605 Table 1. PMSE studies at different frequencies. Frequency Location Reference Reflectivity (Bragg scale) [MHz (m)] [m −1 ] 2.78 (53.9) Tromsø, (69 ◦ N) Bremer et al. (1996b) 8–9 (18.8) Vasil’surk, Russia (56 ◦ N) Karashtin et al. (1997) 3.3, 4.9, 7.6 Gakona, Alaska, (62 ◦ N) Kelley et al. (2002) 9, 11, 13. 15 Hankasalmi, Finnland, (62 ◦ N) Ogawa et al. (2003) 49.6 (3.0) Tromsø, (69 ◦ N) Röttger et al. (1990) 2.0·10 −12 50.0 (3.0) Poker Flat (65 ◦ N) Ecklund and Balsley (1981) Kelley and Ulwick (1988) 9.0·10 −15 51.5 (2.9) Resolut Bay (75 ◦ N) Huaman et al. (2001) 53.5 (2.8) Andøya, (69 ◦ N) Inhester et al. (1990) 4.0·10 −12 53.5 (2.8) Svalbard, (78 ◦ N) Röttger (2001) 2.2·10 −14 224 (0.67) Tromsø, (69 ◦ N) Hoppe et al. (1988) 1.5·10 −16 Röttger et al. (1988) Hocking and Röttger (1997) 1.3·10 −15 500 (0.3) Svalbard, (78 ◦ N) Röttger (2001) 5.3·10 −19 933 (0.16) Tromsø, (69 ◦ N) Röttger et al. (1990) 1.2·10 −18 1290 (0.12) Sondrestrom, (67 ◦ N) Cho and Kelley (1992) this new information does not change the general conclusion that the PMSE- and ice particle- existence period and altitude region are identical. 2.2.2 Latitudinal variation Apart from the aforementioned measurements at ∼65–69 ◦ N, PMSE have also been observed as far North as 75 ◦ N and 78 ◦ N (Huaman et al., 2001; Rüster et al., 2001; Lübken et al., 2004) and as far South as 52–54 ◦ N where the echoes are then consequently called ‘mesosphere summer echoes’ or MSE (Czechowsky et al., 1979; Reid et al., 1989; Thomas et al., 1992; Latteck et al., 1999b; Zecha et al., 2003). While a lot of general features like the typical echo strengths and the altitude region of occurrence are independent of the geographic latitude, it has been found that the occurrence rate shows a pronounced gradient with latitude (all values for 1 July): the occurrence rate drops from almost 100% at 78 ◦ N (Lübken et al., 2004), ∼90% at 69 ◦ N (Bremer et al., 2003), to only ∼10–20% at 54 ◦ N (Zecha et al., 2003). This general picture is in line with recent 3d-model results of the thermal and dynamical structure of the mesosphere suggesting colder temperatures closer to the pole and hence a more favorable environment for the formation of ice particles (von Zahn and Berger, 2003). This picture is only ‘disturbed’ by the observations at Resolut Bay (75 ◦ N) where low occurrence frequencies of less than ∼50% have been reported (Huaman et al., 2001). Note though that occurrence frequencies reported from different stations and radars must be considered with some caution since the radars are not identical in design and hence do not have the same absolute sensitivity. In particular, it must be noted that the VHF radar at Resolut Bay operated at a significantly smaller peak transmitted power (i.e., only 12 kW compared to 36 kW in the case of the radars at 69 ◦ N and 54 ◦ N, and even 60 kW in the case of the SOUSY Svalbard radar at 78 ◦ N). Unfortunately, absolutely calibrated volume reflectivities from these radars are only available for some selected case studies (see Sect. 2.3.1) since absolute calibrations are extremely difficult to obtain. Nevertheless, in future experiments every effort should be undertaken to obtain PMSE measurements using calibrated signal strengths, since only this will allow to perform unambiguous comparisons of observations at different sites. PMSE have also been observed at Southern latitudes even though significantly less frequent than in the Northern hemisphere (Balsley et al., 1993, 1995; Woodman et al., 1999; Ogawa et al., 2002; Hosokawa et al., 2004). Possible hemispheric differences in the thermal and dynamical structure have been discussed (Dowdy et al., 2001; Becker and Schmitz, 2003; Siskind et al., 2003; Chu et al., 2003), however, the experimental data base is scarce such that the reason for the apparent hemispheric asymmetry remains to be identified (Lübken et al., 1999). 2.2.3 Long term and short term variations Recently, first results on long term changes of PMSE properties have been reported. Bremer et al. (2003) considered mean PMSE occurrence frequencies for the years 1994 through 2001 and found a significant positive correlation with the solar Ly-α radiation (the Ly-α flux varies by about www.atmos-chem-phys.org/acp/4/2601/ Atmos. Chem. Phys., 4, 2601–2633, 2004
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